H2 chemisorption and consecutive UV stimulated surface reactions on nanostructured MgO

Abstract

MgO nanoparticles obtained by chemical vapour deposition (CVD) were exposed to H₂ and subsequently to UV irradiation and/or molecular oxygen at room temperature. A combined IR/EPR study reveals the role of low coordinated surface sites and anion vacancies in the diverse reaction steps. The hydride groups emerging from the initial H₂ chemisorption processes (heterolytic splitting) play an active role in the consecutive reactions. They provide the electrons which are required for the UV induced formation of surface colour centres and for the production of superoxide anions (redox reaction). Both the colour centres and the superoxide anions are EPR active. The hydroxy groups resulting from H₂ chemisorption do not actively participate in the consecutive reactions. Together with the OH groups formed in the course of colour centre formation they rather play the role of an observer. They undergo specific electronic interactions with both the colour centre and the superoxide anion which are IR inactive (or IR inaccessible) surface species. They may, however, be observed by IR spectroscopy via the specifically influenced OH stretching vibrations. This proves the intimate interplay between IR and EPR spectroscopy as applied to the surface processes under investigation. As a result, two paths were found for the three consecutive surface reaction steps: H₂ chemisorption, colour centre formation and superoxide anion formation. In the first one a single, well defined surface area element is involved, namely a low coordinated ion pair, the cation of which is a constituent of an anion vacancy. In the second path a diffusion controlled intermediate step has to be adopted in which the electron required for the colour centre is transported by an H atom travelling from a hydride group to a remote anion vacancy. In either case there is clear experimental evidence that the finally resulting superoxide anions are complexed by the colour centre cations.